1. Field of the Invention
The present invention relates to a surface mounting-type semiconductor element-mounting printed board.
2. Description of the Related Art
The mounting of a printed board can be grouped into an insertion mounting type and a surface mounting type, but the latter is now more widely used than the former. Further, in the surface mounting type, a bare chip direct mounting is now more widely used than a package element mounting.
In the bare chip mounting process, the number of electrodes per unit area is increased, and thus the bonding portion is finer. Accordingly, the bonding portion in the bare chip mounting is subject to a concentrated thermal stress, to thereby increase the strain at soldered portions and cause fatigue breakages, and as a result, the number of times the life time test, described later, is repeated is, for example, less than 700, and thus the reliability of the bonding portion is undesirably lowered.
To solve the above problem, Japanese Unexamined Patent Publication (Kokai) No. 61-208889 proposes an insulating resin layer 8 for releasing heat in a metal substrate 9, a conductor circuit 7 being formed on the insulating resin layer 8 as seen in FIG. 1. Japanese Unexamined Patent Publication (Kokai) No. 61-295692 discloses, as the above metal substrate, a metal substrate composed of an invar plate and a copper foil and having a high thermal conductivity and low thermal expansion coefficient.
Further, it is known that the following means are effective for solving the above problem; the thermal expansion coefficients of a chip element and a substrate are made as identical as possible, and thus the shear stress due to a difference between the thermal expansion coefficients is made as small as possible. To accomplish this, the thermal expansion coefficient of the substrate 9 must be made as small as possible and approximately the same as that of chip elements or a conductor circuit 7. Moreover, a resin insulating layer 8 having an excellent thermal release is formed between the substrate 9 and the conductor circuit 7, to prevent a storage of heat in the substrate 9 and the conductor circuit 7.
Nevertheless, even if an insulating layer 8 and a conductor circuit 7 are formed on a side of the substrate 9, the substrate 9 is easily warped. Moreover, in such a composite substrate, a warping of the substrate may occur due to a change of atmosphere, if the thickness of the insulating layer 8 is not adequate, and a perpendicular stress is then increased at the bonding portion, and thus the reliability of the bonding portion is poor.